The Trans-Brake acts in much the same manner as a manual clutch. Once the Trans-Brake is engaged, the vehicle can neither roll forward nor backward and as you press on the accelerator the engine can rpm as high as the torque converter stall. Once the Trans-Brake is released, the power is transmitted immediately to the rear tires in much the same manner as a manual clutch but with the parts saving advantage of the torque converter's shock absorbing action.
I love using the TransBrake. Definitely need a good tranny cooler if you plan on using the TBrake a lot since it creates a ton of heat. The old motor had a PI converter that would flash to 3000 off the footbrake. When I put it against the tbrake it went up to 3400...and the boost went up to about 7lbs of boost...now that was fun leaving the line.
The new NA set-up will have a converter from PI that will flash around 4200 off the footbrake and probably be around 4500 on the tbrake.